The invention relates to a method for coding an analog signal occurring with a certain time interval, said analog signal being converted into control codes which can be used for assembling a synthetic signal corresponding to said analog signal. The invention also relates to an apparatus for carrying out such a method. In particular, the invention relates to a method and apparatus for coding speech signals as digital signals having a low bit frequency.
Such a method or apparatus is disclosed by EP-307,122. According to the known method, an analog (speech) signal (after linear predictive coding (LPC)) is successively converted into a pulse signal composed of pulses at equal (time) spacing from one another, the amplitude of said pulses corresponding to the respective instantaneous amplitudes of the analog signal. A series of p second pulse signals is then generated, all of which are composed of only one pulse, of which, however, the position (in the time domain) of said pulse successively increases with respect to the start of the second pulse signal according to the series based on n times the time spacing of the first pulse signal, where n=0 . . . p. Of said second pulse signals, that pulse signal is then selected which approximates best to the first pulse signal. The first pulse signal is then compared with a set of various third pulse signals, all composed of a number of pulses at mutually different spacings and having mutually different amplitudes, but all of which belong to one and the same class and of which the position of the most significant pulse corresponds to the position of the selected second pulse signal. From this set, that third pulse signal is then selected which corresponds most to the first pulse signal. According to the known method, the set of third pulse signals forms part of a group of such sets, each set having its own class as regards the position of the most significant pulse. By selecting the best second (one-) pulse signal, that set (=class) is therefore indicated which has to be searched for correspondence to the first pulse signal. After selecting the most corresponding third pulse signal, the characteristics of said third pulse signal are used as a control code for assembling a synthetic signal corresponding to said analog signal. In the proposed manner, only a limited set of third pulse signals has to be searched for correspondence, instead of all the third pulse signals of all the sets; in other words, only a part (characterized by the relevant class) of a large set has to be searched instead of said set in its entirety.
A drawback of the known method is that it does not fit in with the present GSM (Grouppe Speciale Mobile) practice.